In the fabrication of electronic devices, it is usual to initially grow a semiconductor wafer from silicon. A top side of the semiconductor wafer is then processed to form a plurality of electronic devices, e.g. transistors etc., on the wafer. A backside of the semiconductor wafer is provided with at least one metallisation layer. The semiconductor wafer is then sawed into a plurality of individual sections, or dies, each comprising at least one electronic device. When a die comprising at least one device is to be attached to, for example, a circuit board for use, it is the metallisation layer on the backside of the die which is used for attaching the die. This is commonly achieved by using solder to attach the die to a die attach flag of the board, the die attach flag being manufactured from a conductive material. The die is then in electrical and thermal contact with the board.
In the process of soldering a die onto a die attach flag, it is not unusual to get voids or gas bubbles in the solder joint between the die and the die attach flag. This is undesirable for a number of reasons. The presence of voids can affect the thermal and electrical performance of the die, by changing the electrical and thermal path within the solder joint. In normal operation of the one or more electronic devices on the die, heat is generated. This is dissipated by transfer to the die attach flag, which comprises a conductive material and acts as a heat sink. The heat is dissipated to the die attach flag via the metallisation layer of the die and the solder joint between the die and the die attach flag. If voids are present in the solder joint, these will affect the dissipation of the heat. This is undesirable, as it may lead to overheating of the electronic devices on the die in operation. In a worst case, this might cause local overheating and damage of a device.
U.S. Pat. No. 6,362,435 describes the modification of a conductor pad formed on a substrate or a printed circuit board, to reduce the problem of void formation on attachment of an electronic device to the conductor pad. The conductor pad comprises a uniform and electrically conductive base layer. A patterned layer is provided on top of the base layer. Either the base layer or the patterned layer is substantially non-wettable (i.e. solder will not adhere thereto), and together the base layer and the patterned layer strips of non-wettable surface areas. When an electronic device is attached to the conductor pad using solder, this will not adhere to the strips, thus providing paths in the solder joint for escape of gases formed in the soldering process. This provides a reduction in the formation of gas voids in the solder. The method described in this document is limited to patterning of substrates and printed circuit boards, and the described solution to the formation of voids, requires a ‘stack-up’ structure, i.e. consisting of two layers.
There remains a need for improving current die solder attach processes, in order to achieve a minimum voiding level in mass production.